1. Field of the Invention
The present invention relates to an integrated circuit structure for a bipolar power device having high current density and a fast diode, and further relates to a manufacturing process for such device.
2. Description of the Prior Art
In bipolar devices, the lifetime of minority carriers strongly influences both the static characteristics and the dynamic characteristics of the device. The affected static characteristics include current capacity, and dynamic characteristics include switching times and energy loss per cycle. As far as static characteristics are concerned, it is known that the current capacity for bipolar transistors is determined by the profiles of the base and emitter regions, known as emitter efficiency. The current capacity is further determined by the thickness of the collector and by the lifetime of minority carriers in the active base region.
For example, for high voltage devices (400 volts or higher), the necessary large collector thickness requires the presence of minority carriers having very long lifetimes. This prevents the minority carriers from recombing along the active base region.
For the same purpose appropriate gettering techniques are used during device fabrication for absorbing metallic impurities. This results in a device having high minority carrier life times at the end of the high temperature thermal processes required during device fabrication. These techniques are basic to the manufacture of high voltage power transistors.
Relating to dynamic transistor characteristics, it is known that the planar technology used to manufacture bipolar power devices allows the integration of a diode between the collector and emitter. This may be done without any change to the manufacturing process flow. Such a diode, typically manufactured under the emitter contact area (emitter metalization) has a breakdown voltage equal to the power transistor's emitter-collector breakdown voltage. It also has very high reverse recovery times, typically ranging from 500 to 1000 nanoseconds due to the high minority carrier lifetimes. This causes large losses during transistor switching and limits its use in high frequency applications above a few tens or hundreds of KHz. In some power applications, such as motor controllers, for example, it is necessary to use diodes having lower reverse recovery times, typically in the 100 to 200 nanosecond range. These fast diodes can be obtained only with low minority carrier lifetimes, typically ranging from 0.5 to 1 microsecond. Naturally, such short lifetimes are incompatible with the high lifetimes necessary for high current capacity of the power transistor.
Thus, the current state of the art does not allow for the integration of fast diodes and bipolar power structures without strongly adversely impacting the transistor's static output characteristics. Currently, for applications requiring collector-to-emitter diodes, external diodes are used.